Method for treating raw-material powder, apparatus for treating raw-material powder, and method for producing object

ABSTRACT

A method for treating a raw-material powder includes forming a layer of the raw-material powder and removing oxide film formed on a surface of the raw-material powder from which the layer has been formed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/373,216 filed Dec. 8, 2016, which claims priority to Japanese PatentApplication No. 2015-241005 filed Dec. 10, 2015, both are herebyincorporated by reference in their entirety.

BACKGROUND Field of the Disclosure

The present disclosure relates to a method and an apparatus for treatinga raw-material powder in which the raw-material powder is treated withplasma.

Description of the Related Art

In recent years, fabrication devices that use the powder bed fusiontechnology (called 3D printers) have been under development. In powderbed fusion, a slice of a raw-material powder is formed, the region ofeach formed slice to be solidified (hereinafter referred to as thesolidifying region) is irradiated with a laser beam or an electron beam(hereinafter referred to as an energy beam) to heat the solidifyingregion, and such slices are stacked to form a three-dimensional object(Japanese Patent Laid-Open Nos. 8-39275 and 10-88201).

It is generally believed that three-dimensional objects fabricated usingpowder bed fusion (hereinafter referred to as powder bed fusionproducts) can be strengthened by increasing the infill by reducing thevoid volume of the structure. Even in slices in which a sphericalraw-material powder is the most closely packed, the interparticle voidvolume per unit volume exceeds 20%. This means that in the state that iscalled sintered, in which particles are fused only at the points ofcontact, there are countless voids between particles.

Japanese Patent Laid-Open No. 8-39275 proposes a powder bed fusiontechnique in which a three-dimensional object is fabricated through theformation of slices in an atmosphere supplied with an inert gas in anevacuated enclosure and heating of solidifying regions of the slicesusing a laser beam. Japanese Patent Laid-Open No. 10-88201 states thatforming a shaped article from a raw-material powder without use ofbinder and compressing the raw-material powder before laser-beam powderbed fusion can increase the infill of the three-dimensional object.

The methods according to these publications for fabricating athree-dimensional object were found to fail to increase the infill ofpowder bed fusion products to sufficiently high levels because thesemethods leave very small voids in the structure of the finishedthree-dimensional object. The present disclosure aims to cure theseshortcomings.

SUMMARY

A method according to an aspect of the disclosure for treating araw-material powder includes forming a layer of the raw-material powderand removing oxide film formed on a surface of the raw-material powderfrom which the layer has been made.

A method according to an aspect of the disclosure for producing anobject includes forming a layer of a raw-material powder, removing oxidefilm formed on a surface of the raw-material powder from which the layerhas been made, and shaping with a beam including solidifying, throughirradiation with an energy beam, the raw-material powder from which theoxide film has been removed.

An apparatus according to an aspect of the disclosure for treating araw-material powder includes an evacuable enclosure, an atmospheregenerator configured to generate, in the enclosure, an atmospherecontaining hydrogen and/or an inert element, a powder container locatedin the enclosure and electrically insulated from the enclosure, aformation unit configured to form a layer of the raw-material powder inthe powder container, and an energizing unit configured to apply avoltage to the layer formed by the formation unit.

According to certain aspects of the disclosure, the amounts of oxidesand foreign substances on the surface of a raw-material powder arereduced through plasma treatment of the raw-material powder. Thisreduces the volume of voids that oxides and foreign substances on thesurface of the raw-material powder leave in the structure of thefinished three-dimensional object. Certain aspects of the disclosuretherefore increase the infill of powder bed fusion products tosufficiently high levels. Furthermore, certain aspects of the disclosuremake powder bed fusion products stronger through the use of araw-material powder treated with plasma.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram for the configuration of an objectproduction system, according to one or more embodiments of the subjectdisclosure.

FIG. 2 is an explanatory diagram for the configuration of a constructioncontainer, according to one or more embodiments of the subjectdisclosure.

FIG. 3 is a flowchart for a process for the production of an object,according to one or more embodiments of the subject disclosure.

FIGS. 4A to 4F are explanatory diagrams for a stacking phase performedwith an object production system, according to one or more embodimentsof the subject disclosure.

FIG. 5 is an explanatory diagram for the configuration of an objectproduction system, according to one or more embodiments of the subjectdisclosure.

DESCRIPTION OF THE EMBODIMENTS

The following describes some embodiments of the disclosure in detailwith reference to the attached drawings. Problems with Known Powder BedFusion Techniques

Objects machined out of a metal ingot produced in a melting furnaceusually have an infill of 99.9% or more.

On the other hand, metal objects fabricated using known powder bedfusion techniques have an infill of at most 99.7% and therefore cannotbe as dense as objects machined out of metal ingots. When the intendeduse is one that requires high surface quality, such as molding dies,objects created using known powder bed fusion techniques are difficultto use directly for that purpose because of their rough surface.

Furthermore, metal objects fabricated using known powder bed fusiontechniques have many pores or voids on their surface or in theirinternal structure. When such known objects are used for constructionpurposes, which require high tensile and bending strength, concernsarise regarding the growth of cracks or fatigue fractures that originatein the pores on the surface and in the internal structure of theobjects.

When used as molding dies, which need to have high surface quality, orfor construction purposes, which require high strength, powder bedfusion products may typically have an infill of 99.9% or more. The lowinfills of objects fabricated using known powder bed fusion techniques,less than 99.7%, combined with the presence of many pores or voids ontheir surface and in their internal structure have been one of thelimiting factors to the widespread use of such objects.

In Embodiment 1, a slice of a raw-material powder is treated with plasmain an argon and hydrogen gas atmosphere, the solidifying region (regionto be solidified) of the slice is then immediately heated with a laserbeam in the argon and hydrogen gas atmosphere, and such slices arestacked. This provides three-dimensional objects that have an infill ofmore than 99.9% with a high frequency and high reproducibility comparedwith known techniques.

This suggests that the low infills observed with known powder bed fusiontechniques are attributable to passivation film or dirt existing on thesurface of the raw-material powder and that plasma treatment removesthem with its physical or chemical effects. Furthermore, the plasmatreatment improves the surface wettability of the raw-material powder byincreasing surface energy, and this may also contribute by preventingair bubbles from being entrained during fusion.

Embodiment 1 Object Production System

FIG. 1 is an explanatory diagram for the configuration of an objectproduction system according to Embodiment 1. FIG. 2 is an explanatorydiagram for the configuration of a construction container. Asillustrated in FIG. 1, the object production system 100 is one that usesthe powder bed fusion technology, therefore what is called a 3D printer.The enclosure 101 is made of stainless steel and is capable ofpreventing the external air from entering its internal space. There is avacuum gauge 208, an example of a detector, connected to the enclosure101. The vacuum gauge 208 detects the pressure in the enclosure 101.

The degassing mechanism 103, an example of an evacuator, is capable ofreducing the pressure in the enclosure 101. The degassing mechanism 103removes air out of the enclosure 101, primarily to reduce the amount ofoxygen in the atmosphere in the enclosure 101. The degassing mechanism103 is a series connection of a dry pump and a turbomolecular pump andis capable of evacuating the enclosure 101 to a degree of vacuum of, forexample, 1×10⁻⁴ Pa.

The degassing mechanism 103 also has an orifice-regulating valve at theconnection with the enclosure 101 with which the orifice size can beregulated. The control unit 200 controls the atmosphere and the degreeof vacuum in the enclosure 101 by operating a gas feed mechanism 102,described hereinbelow, to deliver gases into the enclosure 101 andadjusting this orifice-regulating valve in accordance with the output ofthe vacuum gauge 208.

The gas feed mechanism 102, an example of a feeder, delivers argon gas,an example of an inert gas, and hydrogen gas into the enclosure 101. Thegas feed mechanism 102 is capable of delivering a mixture of argon andhydrogen gases into the enclosure 101 in any proportions of mixing.There may be two separate gas feed mechanisms to deliver argon andhydrogen gases separately.

As illustrated in FIG. 2, the construction container 107, an example ofa powder container, has a vertically movable stacking platform 112inside a construction chamber 109. The lowering mechanism 111 is capableof moving the stacking platform 112 downward in a stepwise manner by anypitch length corresponding to the thickness of the layer 104. In theconstruction chamber 109, treated layers 104′ are stacked as a result ofthe plasma treatment of each layer 104.

There is in a wall of the construction chamber 109 an embeddedresistance heater 137, an example of a heater, capable of heating thelayer 104. There is a temperature sensor 209 on the top surface of thestacking platform 112. The control unit 200 turns on and off the flow ofelectric current into the resistance heater 137 in accordance with theoutput of the temperature sensor 209 to maintain a constant temperatureof the layer 104.

The layer-forming mechanism 105, an example of a formation unit, forms alayer 104 of the raw-material powder in the construction chamber 109 ofthe construction container 107 located in the enclosure 101. The layerof the raw-material powder formed by the layer-forming mechanism 105 isvery thin, having a thickness of 5 μm or more and 200 μm or less. Thelayer 104 of the raw-material powder is thus herein referred to as theslice.

The slice-forming mechanism 105 has a moving unit 133 that moves, asguided by a guide 132, in the direction of the arrow R105 along the topsurface of the construction container 107. The raw-material powder 135is stored in a raw-material reservoir 130 and raised to a level higherthan the top surface of the construction container 107 through a lift ofa baseplate 134. The slice-forming mechanism 105 strikes off theraw-material powder that has emerged on the top surface of theconstruction container 107 while rotating its metal roller 131 in thecounter direction with respect to the top surface of the constructioncontainer 107, thereby forming the slice 104 of the raw-material powder135 in a uniform thickness and a dense structure on the top surface ofthe stacking platform 112. The slice-forming mechanism 105 also forms anew slice 104 of the raw-material powder on treated slice(s) 104′, eachof which is a slice 104 formed in the construction chamber 109 andtreated with plasma.

Construction Container

The raw-material powder, even if its inside is metal, usually is of lowconductivity between its particles because its surface is covered withpassivation film such as oxide film. It has thus been generally believedthat plasma treatment of the raw-material powder through exposure to aplasma arc requires applying an AC voltage to a plate electrode facingthe slice 104. A plate electrode placed above the slice 104, however,blocks the optical path through which the slice 104 is irradiated with alaser beam and collides with the slice-forming mechanism 105 movingalong the top surface of the construction container 107. It has thusgenerally been believed that a plate electrode placed above the slice104 requires a retraction mechanism that physically retracts the plateelectrode from above the slice 104.

The object production system 100, however, repeats forming a slice 104and treating it with plasma as many as several hundreds to thousands oftimes even when constructing an object with a thickness of several tensof millimeters as described hereinafter. Retracting a plate electrodefrom above the slice 104 each time when a slice 104 is formed wouldtherefore lead to a great loss of time. There would also be the problemsof falling contaminants associated with the retraction of the plateelectrode and faults in the retraction mechanism.

The inventors developed a mode of plasma discharge for the plasmatreatment of the slice 104 formed in the construction container 107 thatuses no plate electrode above the slice 104. The inventor first foundthat applying an AC voltage to the slice 104 in an electrically floatingstate with respect to components such as the supporting structure forthe enclosure 101 and the construction container 107 produces a uniformplasma arc across the surface area of the slice 104. The inventorfurther developed this into a mode of plasma discharge in which theenclosure 101 is grounded, the construction container 107 iselectrically isolated from the enclosure 101, and an AC voltage isapplied to an electrode 108 in contact with the slice 104. Such a modeof plasma discharge can be performed without a plate electrode facingthe slice 104 because the slice 104 itself serves as a dischargeelectrode.

As illustrated in FIG. 1, the construction container 107 is located inthe enclosure 101, which is grounded, and is electrically isolated fromthe enclosure 101. The construction container 107 and the stackingplatform 112 are made of insulating materials to avoid plasma formationon their surfaces. The surfaces of the construction container 107 thatcome into contact with the slice 104 are insulating. The electrode 108(energizing unit) that supplies the slice 104 of the raw-material powderwith a voltage that comes from a power supply 113 has an insulatingcover 108 a to avoid plasma formation on its surface.

In Embodiment 1, the slice 104 formed in the construction chamber 109 ofthe insulating construction container 107 is electrically isolated fromthe enclosure 101, and the electrode 108 is used to apply an AC voltageto the slice 104. With the construction container 107 holding the slice104 and electrically isolated from the enclosure 101, an AC voltage isapplied to the electrode 108 lying in contact with the slice 104. Withsuch a configuration, it is possible in Embodiment 1 to generate uniformplasma across the surface area of the slice 104 and rapidly perform auniform plasma treatment of the raw-material powder for the slice 104without needing a plate electrode or any similar component.

Plasma Treatment

As illustrated in FIG. 1, the object production system 100 generatesplasma in the space next to the slice 104 of the raw-material powderformed in the construction container 107 by applying an AC voltage tothe slice 104. The object production system 100 performs the plasmatreatment of a slice 104 formed as a first layer on the stackingplatform 112, and carries out the plasma treatment of slices 104 formedon the treated slice 104′ as second and subsequent layers.

The power supply 113, an example of an energizing unit, and theelectrode 108 apply an AC voltage to the slice 104. The power supply 113applies the AC voltage to the slice 104 of the raw-material powder viathe electrode 108. As illustrated in FIG. 2, the electrode 108 lies incontact with the slice 104 or treated slice 104′ in the constructioncontainer 107. The power supply 113 is also capable of produce a DCvoltage or an AC voltage superposed with a DC voltage. Multiple DCvoltages can be selected from the range of −500 V to +500 V. For the ACvoltage, multiple amplitudes and multiple frequencies can be selectedfrom the ranges of 0 to 2000 V and 10 kHz to 500 kHz, respectively.

The control unit 200 operates the power supply 113, with the enclosure101 fed with hydrogen and/or inert gases and its degree of vacuum being10 Pa or more and less than 10 kPa. This ensures localized formation ofplasma in the space next to the slice 104, providing efficient exposureof the raw-material powder for the slice 104 to plasma and rapid plasmatreatment of the raw-material powder.

During the plasma treatment, the raw-material powder is held in a plasmathat contains hydrogen ions and/or ions of argon, an example of an inertgas, and free electrons. The plasma treatment that uses argon ions andfree electrons is intended to provide the effect called sputtercleaning. The argon ions collide with the surfaces of the particles ofthe raw-material powder and sweep away the adhering oxide film orforeign substances. The free electrons also collide with the surfaces ofthe particles of the raw-material powder, and the surfaces heated tohigh temperatures evaporate away the adhering oxide film or foreignsubstances. As a result, the material for the raw-material powderbecomes exposed from under covers of foreign substances.

On the other hand, the plasma treatment that uses hydrogen ions isprimarily aimed at removing passivation film on the surfaces of theparticles of the raw-material powder through chemical reduction. Thisgives the raw-material powder deoxidized and highly crystallizableparticle surfaces, ensuring that the particles of the raw-materialpowder fuse even at relatively low temperatures and helping them formcrystals with few crystal lattice irregularities.

As illustrated in FIG. 2, the resistance heater 137 may be energizedduring the plasma treatment to additionally heat the slice 104 of theraw-material powder on the stacking platform 112 to make the plasmatreatment more efficient. The laser beam LB may be used as an additionalheat source instead of the resistance heater 137 or in combination withthe resistance heater 137. More specifically, it is possible to heat theslice 104 without melting it by traversing the laser beam LB at reducedintensity or across a broader spot area.

Laser Thermoforming

As illustrated in FIG. 1, the scanning heating mechanism 106 optionallyincludes optical elements such as a condensing lens and a collimatorlens. The light source 110 is a YAG laser oscillator with a power of 500W. The scanning heating mechanism 106 moves a laser beam generated atthe light source 110 using a galvanometer mirror 106 m to scan thesolidifying region of the treated slice 104′ to heat this region with abeam spot of the laser beam LB.

The design data for the three-dimensional object generated using 3D CADsoftware is processed on 2D CAM software into two-dimensional slice datafor multiple solidifying regions the intervals between which correspondto the thickness of the slice 104. The NC program for the scanningheating mechanism 106 converts the two-dimensional data for the specificsolidifying regions into scanning programs for the solidifying region ofeach slice 104 that control simultaneous relative movements in twohorizontal directions. The control unit 200 controls the scanningheating mechanism 106 as directed by the scanning programs for eachsolidifying region to scan predetermined paths on the plane of theconstruction container 107 with a laser beam LB.

The scanning heating mechanism 106, an example of a beam emitter,irradiates the treated slice 104′, or the slice 104 treated with plasma,with a laser beam LB, an example of an energy beam. The treated slice104′, forming the top surface of the construction chamber 109 of theconstruction container 107, is heated by a laser beam emitted from thescanning heating mechanism 106. The laser beam melts the treated slice104′ and solidifies it with the lower solid structure almost in amoment. As a result, a region of the treated slice 104′ as the topsurface of the construction chamber 109 is melted and solidified. Theplasma treatment and the energization of the resistance heater 137 maycontinue during this laser thermoforming phase so that the treated slice104′ remains at a high temperature. Maintaining the treated slice 104′at a high temperature leads to more efficient laser thermoforming andtherefore enables rapid laser thermoforming even when the power of thelaser beam is low and, furthermore, gives the resulting object 301 amore uniform structure by ensuring that the treated slices 104′ aremelted in a more consistent manner.

Note that heating with a laser beam in this phase of laser thermoformingcan be used to promote the plasma treatment of the raw-material powderfor the treated slice 104′. The plasma treatment of the raw-materialpowder therefore does not need to be 100% completed in the above phaseof plasma treatment. For example, it is possible to suspend thetreatment at a roughly 80% progression in the phase of plasma treatmentand achieve 100% of the intended level of treatment in the phase oflaser thermoforming before melting the raw-material powder. Thisaccelerates the production of the object 301 by reducing the timerequired for plasma treatment.

Process for the Production of an Object

FIG. 3 is a flowchart for a process for the production of an object.FIGS. 4A to 4F are explanatory diagrams for a stacking phase performedwith an object production system. As illustrated in FIG. 1, the controlunit 200 has a CPU 205, a RAM 206, and a ROM 207. By storing processcontrol programs called from the ROM 207 in the RAM 206, the CPU 205functions as a process controller for the object production system 100.The user instructs the system to start the process via the console 201.

As illustrated in FIG. 3, in response to the process startup command,the control unit 200 operates the degassing mechanism 103 to evacuatethe enclosure 101 (S11). After the pressure in the enclosure 101 hasreached 1×10⁻² Pa, the gas feed mechanism 102 is operated to start gasfeeding and adjust the pressure in the enclosure 101 to asub-atmospheric process pressure (S12). In the atmosphere generationphase composed of steps S11 and S12, as illustrated in FIG. 1, theenclosure 101 is evacuated to a first pressure, and then argon andhydrogen gases are delivered into the enclosure 101 to create, in theenclosure 101, an atmosphere of a second pressure that is higher thanthe first pressure and lower than atmospheric pressure.

As illustrated in FIG. 3, after the pressure in the enclosure 101 hasreached the second pressure (e.g., 5 kPa), the control unit 200 operatesthe slice-forming mechanism 105 to form a slice 104 of the raw-materialpowder (S13). As illustrated in FIG. 4A, in the lowering phase, thelowering mechanism 111 is operated to lower the stacking platform 112 tomake a space inside the construction container 107 for the slice 104 tobe formed in. As illustrated in FIG. 4B, in this formation phase, or instep S13, the slice-forming mechanism 105 is operated to form a slice104 of the raw-material powder on the stacking platform 112.

As illustrated in FIG. 3, the control unit 200 operates the power supply113 to start plasma generation (S14) and continues the plasma treatmentof the slice 104 until a specified time elapses (NO in S15). Asillustrated in FIG. 4C, in this plasma treatment phase composed of stepsS14 and S15, the slice 104 of the raw-material powder on the stackingplatform 112 is treated with plasma. In the plasma treatment phaseplasma is generated in a low-oxygen atmosphere containing argon andhydrogen gases through the application of an AC voltage to the slice 104with the power supply 113. The plasma treatment phase is performed in anatmosphere of a sub-atmospheric pressure (preferably 10 Pa or more andless than 10 kPa) generated in the enclosure 101. The plasma treatmentphase is carried out while maintaining a constant temperature of theslice 104 by heating the slice 104 using the resistance heater 137.

As illustrated in FIG. 3, after the completion of the plasma treatmentPN of the slice 104 (YES in S15), the control unit 200 controls thescanning heating mechanism 106 and the light source 110 to perform thelaser thermoforming of the treated slice 104′ (S16). In this phase ofshaping with a beam, or in step S16, the scanning heating mechanism 106and the light source 110 are operated to perform laser thermoforming tomelt and solidify the treated slice 104′ on the stacking platform 112 asillustrated in FIG. 4D. In the phase of shaping with a beam thesolidifying region of the treated slice 104′ is irradiated with anenergy beam for solidification. The phase of shaping with a beam isperformed while plasma is generated in a low-oxygen atmospherecontaining argon and hydrogen gases through the application of an ACvoltage to the treated slice 104′.

As illustrated in FIG. 3, after the completion of the laserthermoforming of one treated slice 104′ (S16), the control unit 200turns off the power supply 113 to stop the plasma generation PS on thetreated slice 104′ (S17). The control unit 200 repeats the formation ofa slice (S13), plasma treatment (S14 and S15), laser thermoforming(S16), and stopping plasma generation (S17) until reaching the stackingcount (number of layers stacked) for the completion of the object 301(NO in S18). In the second lowering phase, the lowering mechanism 111 isoperated to lower the stacking platform 112 to make a space on theentire treated slice 104′, including the unsolidified portion, for a newslice 104 of the raw-material powder to be formed in as illustrated inFIG. 4E. In the second formation phase, the slice-forming mechanism 105is operated to form the second slice 104 of the raw-material powder asillustrated in FIG. 4F. The new slice 104 is formed on the treated slice104′ on the bottom surface of the construction container 107.

As illustrated in FIG. 3, the object production system 100 repeats theformation of a slice, plasma treatment, and laser thermoforming toconstruct a three-dimensional object 301 as a stack of the solidifyingregions of treated slices 104′. After reaching the stacking count forthe completion of the object 301 (YES in S18), the control unit 200stops gas feeding (S19) and introduces the outside air into theenclosure 101 (S20). Then the control unit 200 tells the user via thedisplay 202 that the object (shaped article) 301 is ready to be takenout.

Making Objects from Different Materials

Using this object production system 100, some objects 301 werefabricated with different materials for the raw-material powder,different plasma treatment conditions, and different laser thermoformingconditions as in Examples 1, 2, and 3. The resulting objects 301 weretested for density.

Example 1

In Example 1, a raw-material stainless steel powder was subjected toplasma treatment/laser thermoforming under the following conditions.

Plasma Treatment Conditions

Pressure in the enclosure 101: 6.66 kPa

Feed gas: Argon gas

Raw-material powder: A water-atomized raw-material SUS613 powder havinga particle diameter of 7 μm

Thickness of the slice 104: 20 μm

Voltage applied: AC voltage 1 kV, frequency 100 kHz

Duration of treatment: 1 minute

Laser Thermoforming Conditions

Region melted: A square region measuring 25 mm wide and 25 mm long onthe slice 104

Stacking count: 5000

Stacking height: 100 mm

The infill of the object 301 obtained in Example 1 as measured by theArchimedes method was 99.9% or more. Through this it was found that theconditions used in Example 1 provide objects 301 more dense thanstainless steel objects fabricated using the ordinary powder sinteringmethod.

Example 2

In Example 2, a raw-material titanium powder was subjected to plasmatreatment/laser thermoforming under the following conditions.

Plasma Treatment Conditions

Pressure in the enclosure 101: 13.3 Pa

Feed gas: A mixed gas containing 50% argon gas+50% hydrogen gas (% bythe number of molecules)

Raw-material powder: A water-atomized raw-material Ti powder having aparticle diameter of 50 μm

Thickness of the slice 104: 100 μm

Voltage applied: AC voltage 20 kV, frequency 7 kHz

Duration of treatment: 3 minutes

The powder container 107 was preheated using the resistance heater 137and controlled to maintain the slice 104 and the object 301 at 400° C.to accelerate the hydrogen reduction of passivation film.

Laser Thermoforming Conditions

Region melted: A square region measuring 25 mm wide and 25 mm long onthe slice 104

Stacking count: 500

Stacking height: 50 mm

The infill of the object 301 obtained in Example 2 as measured by theArchimedes method was 99.9% or more. Through this it was found that theconditions used in Example 2 provide objects 301 more dense thantitanium objects fabricated using the ordinary powder sintering method.

Example 3

In Example 3, a raw-material aluminum powder was subjected to plasmatreatment/laser thermoforming under the following conditions.

Plasma Treatment Conditions

Pressure in the enclosure 101: 1.0 kPa

Feed gas: 100% hydrogen gas

Raw-material powder: A gas-atomized raw-material Al powder having aparticle diameter of 80 μm

Thickness of the slice 104: 100 μm

Voltage applied: AC voltage 1.5 kV, frequency 100 kHz

Duration of treatment: 3 minutes

The powder container 107 was preheated using the resistance heater 137and controlled to maintain the slice 104 and the object 301 at 400° C.to accelerate the hydrogen reduction of passivation film.

Laser Thermoforming Conditions

Region melted: A square region measuring 5 mm wide and 5 mm long on theslice 104

Stacking count: 50

Stacking height: 5 mm

The infill of the object 301 obtained in Example 3 as measured by theArchimedes method was 99.9% or more. The object oxygen level as measuredby thermal fusion was less than 0.1%. Through this it was found that theconditions used in Example 3 provide objects 301 more dense and purerthan aluminum objects fabricated using the ordinary powder sinteringmethod.

Advantages of Embodiment 1

In Embodiment 1, the plasma treatment of the raw-material powder for theslice 104 is performed in a low-oxygen atmosphere, and this prevents theoxidization of the raw-material powder through which passivation film isformed. Furthermore, the atmosphere in which the raw-material powder forthe slice 104 is subjected to plasma treatment contains argon. The argonions and electrons collide with the surface of the raw-material powderand remove and heat the adhering foreign substances, leaving highlycrystalline surfaces. The atmosphere for the plasma treatment of theraw-material powder for the slice 104 also contains hydrogen, and thisaccelerates the hydrogen reduction of the oxide film existing on thesurface of the raw-material powder through which passivation film iseliminated. In this way, this embodiment solves the infill issue withpowder bed fusion and enables the construction of objects with higherdensity, for example an infill of 99.9% or more, superior surfacecharacteristics, and higher strength.

In Embodiment 1, the plasma is generated at the surface of the slice 104interfacing with the gas in the atmosphere, through the application ofan AC voltage to the slice 104. When the electrode 108 comes intocontact with and applies an AC voltage to the slice 104 or treated slice104′ in the construction container 107, the slice 104 behaves as adischarge electrode. No electrode for applying an AC voltage is neededabove the slice 104. No component that assists discharge, such as aplate electrode or a coil, is needed above the slice 104. There remainsplenty of space above the slice 104 in which nothing interferes with theirradiation of the slice 104 with a laser beam LB and the formation ofthe slice 104 by the slice-forming mechanism 105.

In Embodiment 1, the construction container 107 in which the slice 104is subjected to an AC voltage is made of an insulating material. Thisensures localized formation of plasma on the surface of the slice 104interfacing with the gas in the atmosphere with no plasma occurring onthe construction container 107. The localized formation of plasma onthis surface of the slice 104 leads to highly uniform plasma treatmentacross the entire surface of the slice 104. The argon and hydrogen ionsin the plasma strike the slice 104 in an efficient manner and rapidlyremove materials covering the surface of the raw-material powder such asoxides, passivation film, and dirt. As a result, the plasma treatment inthis embodiment is rapid and highly efficient.

In Embodiment 1, the formed slice 104 of the raw-material powder issubjected directly to the plasma treatment. The treated slice 104′ cantherefore be obtained with any intended level of treatment by extendingthe duration of plasma generation. It is possible to precisely controlthe level of plasma treatment of the treated slice 104′ by adjusting theduration of plasma generation.

In Embodiment 1, the plasma treatment is performed in an atmosphere of asub-atmospheric pressure generated in the enclosure 101. The plasmatherefore remains in a stable state compared with that in a plasmatreatment performed in an atmosphere of atmospheric or higher pressure.

In Embodiment 1, the plasma treatment is performed in an atmosphere of apressure of 10 Pa or more and less than 10 kPa generated in theenclosure 101. The plasma treatment of the raw-material powder istherefore faster than it would be at lower degrees of vacuum.

In Embodiment 1, the enclosure 101 is evacuated to a first pressure, andthen an inert element is delivered into the enclosure 101 to create anatmosphere of a second pressure that is higher than the first pressureand lower than atmospheric pressure. Much oxygen has therefore beenremoved from the atmosphere in which the plasma is generated, and thisprevents the atmosphere during the plasma treatment from beingcontaminated by contaminants contained in the raw-material powder suchas oxygen, organic substances, and water.

In Embodiment 1, the construction container 107 in which the slice 104is formed and treated with plasma is electrically isolated from theenclosure 101. Thus little current leaks when the slice 104 is subjectedto an AC voltage, and the consequent localization of plasma on thesurface of the slice 104 interfacing with the gas in the atmosphereleads to efficient plasma treatment.

In Embodiment 1, the plasma treatment is performed while the slice 104is heated using a heater to maintain a constant temperature of the slice104. This leads to faster plasma treatment by elevating the temperatureof the raw-material powder and ensures more consistent levels oftreatment of the raw-material powder for all layers stacked, from thefirst to the last.

In Embodiment 1, the raw-material powder is metal particles formed bywater atomization. The raw-material cost is therefore lower than withthe use of metal particles formed using gas atomization. In general,metal particles formed by water atomization are covered with thickpassivation film. In this embodiment, however, the passivation film iseliminated through reaction with hydrogen ions in the plasma state, andthis makes the plasma treatment of the raw-material powder efficient.

In Embodiment 1, the solidifying region of the treated slice 104′ isirradiated with a laser beam LB for solidification. The laser beam,unlike an electron beam, is not attenuated or scattered even when theinside of the enclosure 101 is not a high degree of vacuum. It istherefore possible to heat the treated slice 104′ in an efficient mannereven at a low degree of vacuum of 10 Pa or more and less than 10 kPa.

In Embodiment 1, the plasma-treated slice 104′ immediately proceeds tolaser thermoforming without being exposed to the outside air. The plasmatreatment continues during the laser thermoforming. Most of oxides andforeign substances have therefore been removed from the surface of theraw-material powder at the time of laser thermoforming, allowingefficient powder bed fusion.

Crystal Structure of the Object

In Embodiment 1, crystals repeatedly grow upward on the core crystalstherebeneath to form an object 301 in the same way as in the growth ofsingle crystals in the method called zone melting. The object 301,composed of solid layers each formed through plasma treatment of a slice104 of the raw-material powder and subsequent melting and solidificationof the treated slice 104′ with a laser beam LB, therefore has a crystalstructure in which crystals have grown in parallel in the direction fromthe planes on the first layer side to the planes on the last layer side.Furthermore, the object 301 is free of oxides and foreign substancesinside as well as on its surface.

As mentioned above, in Embodiment 1, the infill of the object 301 asmeasured by the Archimedes method is 99.9% or more. There has hithertobeen no technology that solidifies a metal powder to such a high infill.One reason for the high infill is that the plasma treatment removesoxides and foreign substances from the raw-material powder in a highlyefficient manner. Another reason is that the moderate degree of vacuumin the enclosure 101 allows a sufficient amount of argon ions to besaved for the plasma treatment. Treating a finished object 301 withplasma, for example, would leave large amounts of oxides and foreignsubstances inside because the argon ions in the plasma would only strikethe surface of the object 301 without reaching the inside of the object301. Plasma treatment of each slice, if insufficient, would also leaveoxides and foreign substances inside the object 301. In Embodiment 1,each slice is treated with plasma in a specific plasma atmosphere, andthis ensures that the inside of the slice is treated with the plasma aswell as the surface. The resulting object 301 has few voids associatedwith oxides and foreign substances even inside.

Embodiment 2

The examples in Embodiment 1 were ones in which alloy (stainless steel)and pure metal (titanium) objects were produced. In Embodiment 2,examples are described in which nitrided alloy (stainless steel) andcarburized metal (silicon) objects were produced using the objectproduction system 100 illustrated in FIG. 1.

Example 4

As illustrated in FIG. 1, feeding a nitrogen-containing substance viathe gas feed mechanism 102 during the plasma treatment in the objectproduction system 100 initiates plasma nitriding, a reaction throughwhich nitrogen ions generated in the plasma are taken in theraw-material powder. Nitrogen ions accelerated in the plasma enter anddiffuse through the surface of the raw-material powder that has beenheated, cleaned, and activated in the plasma into the inside.

In Example 4, hydrogen gas as a hydrogen-containing substance andnitrogen gas as a nitrogen-containing substance were fed via the gasfeed mechanism 102. Operating the object production system 100 in thesame way as in Embodiment 1 in this state initiated plasma nitriding ofthe raw-material powder instead of plasma treatment. As in Embodiment 1,laser thermoforming followed the plasma treatment (plasma nitriding) ofthe raw-material powder to shape the nitrided powder material into athree-dimensional object.

As illustrated in FIG. 3, the plasma treatment of the raw-materialpowder (S14 and S15) and the laser thermoforming of a slice (S16 andS17) were repeated in a mixed atmosphere containing hydrogen andnitrogen gases to construct an object 301. In Example 4, stainless steelparticles were subjected to plasma nitriding/laser thermoforming underthe following conditions.

Plasma Nitriding Conditions

Pressure in the enclosure 101: 13.3 kPa

Feed gas: A mixed gas containing nitrogen and hydrogen gases in a gasmixing ratio of 1:1

Raw-material powder: Water-atomized stainless steel (SUS613) particleshaving a particle diameter of 7 μm

Thickness of the slice 104: 20 μm

Voltage conditions: AC voltage 1 kV, frequency 100 kHz

Duration of treatment: 3 minutes

Laser Thermoforming Conditions

Region melted: A square region measuring 25 mm wide and 25 mm long onthe treated slice (nitrided slice)

Stacking count: 2000

Height of the object 301: 40 mm

The object 301 obtained in Example 4 was analyzed for nitrogenconcentration using XPS (x-ray photoelectron spectroscopy), with theresult that the nitrogen concentration was 12% (percentage by the numberof atoms). The infill of the object 301 as measured by the Archimedesmethod was 99.9%. The hardness of the object 301 as measured using aVickers hardness tester was HV 2200.

Example 5

Feeding a carbon-containing substance via the gas feed mechanism 102during the plasma treatment in the object production system 100illustrated in FIG. 1 initiated plasma carburizing, a reaction throughwhich carbon ions generated in the plasma were taken in the raw-materialpowder. Carbon ions accelerated in the plasma entered and diffusedthrough the surface of the raw-material powder that had been heated,cleaned, and activated in the plasma into the inside.

In Example 5, hydrogen gas as a hydrogen-containing substance, argon gasas a substance containing an inert gas, and methane gas as acarbon-containing substance were fed via the gas feed mechanism 102.Operating the object production system 100 in the same way as inEmbodiment 1 in this state initiated plasma carburizing of theraw-material powder instead of plasma treatment. As in Embodiment 1,laser thermoforming followed the plasma treatment (plasma carburizing)of the raw-material powder to shape the carburized powder material intoa three-dimensional object.

As illustrated in FIG. 3, the plasma treatment of the raw-materialpowder (S14 and S15) and the laser thermoforming of a slice (S16 andS17) were repeated in a mixed atmosphere containing hydrogen, argon, andmethane gases to construct an object 301. In Example 5, a raw-materialsilicon powder was subjected to plasma carburizing/laser thermoformingunder the following conditions.

Plasma Carburizing Conditions

Pressure in the enclosure: 13.3 kPa

Feed gas: A mixed gas containing methane, hydrogen, and argon gases in agas mixing ratio of 1:2:1

Raw-material powder: A water-atomized raw-material silicon powder havinga particle diameter of 5 μm

Thickness of the slice: 40 μm

Voltage conditions: AC voltage 20 kV, frequency 100 kHz

Duration of treatment per layer: 5 minutes

Laser Thermoforming Conditions

Region melted: A square region measuring 25 mm wide and 25 mm long onthe treated slice (carburized slice)

Stacking count: 100

Stacking height: 4 mm

In Example 5, heating was performed throughout the plasmacarburization/laser thermoforming phases using the resistance heater 137to maintain the temperature of the slice 104 and the object 301 at 800°C. Maintaining the slice 104 and the object 301 at a high temperaturemade the reaction between carbon and silicon even faster. In Example 5,the plasma carburization continued while the treated slice 104′ wasirradiated with a laser beam for laser thermoforming. This ensured thatnot only the slice 104 but also the object 301 continued to be dopedwith the element carbon.

The object 301 obtained in Example 5 was analyzed for carbonconcentration using an XPS (x-ray photoelectron spectroscopy)instrument, with the result that the carbon concentration was 11%(percentage by the number of atoms). The infill of the object 301 asmeasured by the Archimedes method was 99.2%. The hardness of the object301 as measured using a Vickers hardness tester was HV 3000.

Embodiment 3

In Embodiment 1, laser thermoforming was used to melt and solidifytreated slices. In Embodiment 3, electron beam thermoforming is used tomelt and solidify treated slices.

Object Production System

FIG. 5 is an explanatory diagram for the configuration of an objectproduction system according to Embodiment 3. As illustrated in FIG. 5,an object production system according to Embodiment 3 uses an electronbeam instead of a laser beam to thermoform each treated slice but hasthe same configuration and conducts the same plasmatreatment/thermoforming phases as one according to Embodiment 1 exceptfor this. Any components in FIG. 5 equivalent to those in Embodiment 1are given the same reference numerals as in FIG. 1 and are not describedin detail hereinbelow.

The object production system 300 is one that uses the powder bed fusiontechnology, therefore what is called a 3D printer. The degassingmechanism 103 evacuates the enclosure 101. The gas feed mechanism 102delivers gases into the enclosure 101. The object production system 300can also be used as a raw-material powder treatment system, i.e., asystem that only treats a raw-material powder with plasma and produces atreated powder, as in Embodiment 1.

The electron beam heating device (device that performs heating using anelectron beam) 306 generates and traverses an electron beam to heat thesolidifying region of the slice 104 with a spot of the electron beam inaccordance with input data. The electron beam control unit 310 is acontroller that controls the generation and traversing of an electronbeam by the electron beam heating device 306.

Degree of Vacuum in Electron Beam Thermoforming

The object production system 300 according to Embodiment 3 may need tohave a degree of vacuum of 10⁻¹ Pa or less inside the enclosure 101during the electron thermoforming phase because the electron beam isscattered by gas molecules existing in the enclosure 101. For thispurpose, the object production system 300 according to Embodiment 3first performs the plasma treatment while maintaining a degree of vacuumof 100 Pa by feeding a mixed gas containing carbon and hydrogen gases.The system thereafter stops feeding the mixed gas and conducts electronbeam thermoforming after restoring the degree of vacuum to 10⁻¹ Pa.

Other Embodiments

The methods and apparatuses according to certain aspects of thedisclosure for treating a raw-material powder or producing an object arenot limited to any specific configurations, forms of components, numericconditions, or controls described in Embodiments 1 to 3. It is possibleto implement these methods and apparatuses in other embodiments in whichthe configurations of Embodiments 1 to 3 have been partially or totallyreplaced with equivalent components.

The specific voltage and pressure conditions in Examples 1, 2, 3, and 4may be adjusted according to parameters such as the size of theconstruction container 107, the size of the raw-material powder, and thethickness of the slice 104. For example, although in Examples 1, 2, 3,and 4 each slice 104 was subjected only to an AC voltage, the AC voltagemay be superposed with a negative DC voltage to increase the velocity atcollision of positive ions for enhanced heating of the slice 104. Thesubstance used to supply hydrogen to the atmosphere in the enclosure 101may be hydrogen gas, ammonia gas, or a non-methane hydrocarbon gas, forexample.

In Embodiment 1, the proportions of hydrogen and argon gasses in themixed gas remain constant during the application of a voltage to eachslice 104 for plasma generation. However, their proportions may bedifferent between the initial, intermediate, and terminal stages ofplasma generation.

In Embodiment 1, a resistance heater 137 is used for heating during theplasma treatment. However, heaters are not the only possible way of theheating during the plasma treatment. It is possible to pass currentthrough the slice to generate Joule heat. The AC voltage applied to theslice 104 may be superposed with a negative bias voltage in the presenceof argon gas to induce what is called sputter heating.

The examples in Embodiment 1 were ones in which an object was made froman alloy or pure metal like stainless steel or titanium, and theexamples in Embodiment 2 were ones in which an object was made from anitrided or carburized metal like nitrided stainless steel or carburizedsilicon. Some embodiments of the disclosure may, however, produceobjects of alloys or pure metals other than stainless steel and titaniummentioned in the examples in Embodiment 1 (e.g., silicon).Alternatively, the object may be made from an oxidized metal. The plasmatreatment of the raw-material oxidized-metal powder may be performed inan atmosphere that only contains an inert gas, without hydrogen, so asnot to reduce the raw-material powder into pure metal.

In the examples described in Embodiment 1, a raw-material metal powderwas fully melted and recrystallized in stacking solidifying regions. Theheating temperatures for the treatment and laser thermoforming of theraw-material powder in Embodiment 1, however, may be chosen such thatparticles of the raw-material metal or oxidized metal powder, which havea high melting point, are sintered together.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the disclosure is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. An apparatus for treating a raw-material powderin an object production system that uses powder bed fusion, theapparatus comprising: an evacuable enclosure; atmosphere generatorconfigured to generate, in the enclosure, an atmosphere containinghydrogen and/or an inert element; a powder container located in theenclosure and electrically insulated from the enclosure; a forming unitconfigured to form a layer of the raw-material powder in the powdercontainer; and an energizing unit configured to generate plasma in saidatmosphere by applying an AC voltage to the layer formed by the formingunit, wherein the energizing unit has an electrode arranged to come intocontact with the layer and apply the AC voltage to the layer in thepowder container.
 2. The apparatus according to claim 1 for treating araw-material powder, wherein a surface of the powder container thatcomes into contact with the layer is insulating.
 3. The apparatusaccording to claim 1 for treating a raw-material powder, wherein theenergizing unit capable of superposing the AC voltage with a negative DCvoltage.
 4. The apparatus according to claim 1 for treating araw-material powder, wherein the apparatus further comprising a heater,as a component of the powder container, arranged to heat the layer. 5.An object production system that uses powder bed fusion, the objectproduction system comprising: an apparatus for treating a raw-materialpowder; wherein the apparatus includes: an evacuable enclosure;atmosphere generator configured to generate, in the enclosure, anatmosphere containing hydrogen and/or an inert element; a powdercontainer located in the enclosure and electrically insulated from theenclosure; a forming unit configured to form a layer of the raw-materialpowder in the powder container; and an energizing unit configured togenerate plasma in said atmosphere by applying an AC voltage to thelayer formed by the forming unit, wherein the energizing unit has anelectrode arranged to come into contact with the layer and apply the ACvoltage to the layer in the powder container, and a beam emitterconfigured to irradiate, with an energy beam, the layer that has beentreated by the apparatus.
 6. An object constructed by powder bed fusionaccording to claim 5, comprising a crystal structure in which a crystalhas grown in a stacking direction of layers, with an infill as measuredby an Archimedes method being 99.9% or more.
 7. The system according toclaim 5 for producing an object, wherein the electrode arranged to comeinto contact with the layer is configured to not interfere with theirradiation of the layer by the beam emitter.